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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

452
ALASKAN GLACIER STUDIES
the river itself, and there are few finer river trips in the world than that from Chitina River southward through the Copper River canyon. The junior author had the opportunity of making this in 1910, when steamboats were running on the Copper.
The present grade of the river in this upper portion of the canyon ranges between 6 and 8 feet to the mile.   The river is an anastomosing stream, divided into scores of
shallow, braided channels on a narrow valley bottom of glacial outwash gravels. The only exception to this is the short stretch of Wood Canyon, where the river covers the whole bottom of a single, deep gorge. Copper River is heavily laden with glacial debris from, the Wrangell, Chugach, and Alaska Ranges and the bottom of the canyon is being built up, so that the channels are continually shifting. One day in August, 1910, for example, a steamboat upon which the junior author ascended and descended the Copper River for SO miles was obliged, on the return trip, to seek new channels at several points where deposition had shallowed the channels which were navigable earlier the same day.
The portion, of the Copper River cany on between Chitina and Tasnuna River has been intensely glaciated. The walls are much oversteepened (Pig. 70) and the U-shape (PI.
Miles
FIG. 69.   COEPBB RIVER CANYON. (After U. S. Geological Survey.)
CLXXIX) is well developed. The oversteepening results in modern avalanches, as in a case just south of Cleave Creek. There are numerous hanging valleys (see h, Fig. 70), which are at elevations of from 300 to 400 feet for the larger tributaries, increasing to an extreme of 1500 to 2000 feet in the case of a small hanging valley;, just north of Dewey Creek. The post-glacial gorges cut in the lips of these hanging valleys.